Triacetylcellulose (hereinafter abbreviated as TAC) has been conventionally used as a support of roll-like silver halide photosensitive materials. This is because a core set curl formed in a cartridge (a magazine) can be striken off during a development processing. The core set curl (curl in the film's long direction) means a phenomenon that, after a flat film is wound in its longer direction on a roll and is removed from the roll, the film is still apt to become a vortex in form.
Recently, cartridge miniaturization has been desired. However, it has been difficult to make a TAC support thinner than 115 .mu.m, because of its weak mechanical strength.
On the other hand, a polyester support that is formed from polyethyleneterephthalate (hereinafter abbreviated as PET) excels in mechanical strength and is suitable for making a thin support. But it has the drawback that the core set curl cannot be recovered. With regard to a method of removing this drawback from the polyester support, U.S. Pat. No. 4,141,735 describes a method of preventing the core set curl, which method comprises subjecting a polyester-series polymer to heat processing at its glass transition temperature (hereinafter referred to as Tg) or a temperature lower than the Tg (hereinafter referred to as "below Tg annealing," abbreviated as BTA).
A silver halide photographic light-sensitive material generally has a support on which are coated light-sensitive layers that contain a light-sensitive silver halide dispersed in gelatin. These layers shrink at a low humidity condition and cause a curl in the film's width direction. Such a curl in the width direction causes scratches due to friction of a film pressure plate with the back surface of the film during handling of the film in a camera, which results in fatal damage to the film. The curl in the film's width direction (transverse curl) is a phenomenon that a flat film, coated with photosensitive layers on one surface of the film and a back layer on another surface thereof, warps in its width direction, making the photosensitive-layer-coated side become the inner side, to form a round curved surface like a gutter.
There have been known several methods for imparting a reverse curl in advance (a curl formed in a reverse direction in order to make the emulsions-coated-side of the support become convex; it is referred to as reverse curl in this specification) to a polyester base, to strike off such a reverse curl in the width direction. This "curl" has the effect of reducing the transverse curl. For example, JP-A ("JP-A" means a published unexamined Japanese Patent Application) No. 131550/1989 discloses a method wherein the reverse curl is imparted by making a difference in temperature between the surface side and the back side of the film support during its stretching film-forming. This method is suitable for a thick support of 150 .mu.m or more, because it is easy to make a difference in temperature between the surface side and the back side of the support.
However, it has been difficult to apply such a method to a thin support of 100 .mu.m or less, as used in the present invention, since it is difficult to make a difference in temperature between the surface side and the back side of itself. Further, other methods, wherein different layers are laminated, are also known, as described in JP-A No. 123420/1975 and U.S. Pat. No. 4,198,458. However, these methods have a drawback in that the mechanical strength is apt to be lowered.